Airbag passive restraint systems for passenger vehicles are known. These systems typically include a firing circuit having a squib and two normally open inertia switches, all connected in series across a source of electrical energy. The two inertia switches are know in the art as the safing sensor and the front sensor. The safing sensor is located in the passenger compartment and the front sensor is located near the front of the vehicle. Upon closure of the two inertia switches, as occurs during a vehicle crash, electrical current of sufficient magnitude and duration passes through the squib so as to ignite the squib. The squib, when ignited, ignites a combustible gas generating composition or pierces a container of pressurized gas which results in inflation of the airbag.
If the firing circuit in an airbag restraint system is non-operative, the airbag would not be deployed upon the occurrence of a crash condition. Such non-operative conditions, for example, include (i) an open circuit in the series connection of the squib and the two inertia switches, or (ii) a short circuited squib. Also, a non-operative condition exists if excessive impedance exists in connections between firing circuit components so that upon closure of the inertia switches, the squib does not draw enough current to fire the squib.
Diagnostic test circuits have been developed to monitor the operativeness of the firing circuit portion of the airbag passive restraint system. If a non-operative condition is detected in the firing circuit, the diagnostic circuits alert the vehicle operator by lighting a warning lamp. Such diagnostic test circuits are disclosed in U.S. Pat. No. 4,835,513 to McCurdy et al., and in U.S. Pat. No. 4,825,148 to McCurdy et al., both assigned to the assignee of the present invention, both of which are hereby fully incorporated herein by reference. Both the '513 patent and the '148 patent are directed to vehicle restraint systems having only one airbag, preferably on the driver side.
These known diagnostic circuits separately monitor certain parameters of various components in the firing circuit, including the two inertia switches, the squib, and the storage capacitor. The diagnostic circuits monitor for both snort and open circuits in the firing circuit and perform tests to determine if resistance or capacitance values of firing circuit components are within predetermined limits. Upon detection of an unacceptable condition in the firing circuit, the circuit alerts the vehicle operator by lighting an indicator lamp located in the passenger compartment. Also, the diagnostic circuits disclosed in the '513 and the '148 patents provide for recording a detected out-of-tolerance condition in an electrically erasable programmable read only memory ("EEPROM") for later analysis by a service technician.
In a passive restraint system having both driver's side and passenger's side airbags, two firing circuits are typically used. In a dual airbag arrangement, it is contemplated that two firing circuits will be used in which certain firing circuit components are shared or connected in parallel. Such an arrangement improves the overall reliability of a dual airbag system. For example, it is contemplated that the front sensors in a dual airbag system can be connected in parallel so as to provide a redundant circuit path between the squibs and the power supply. Even though certain parallel connections are present in the two firing circuits, it is desirable to separately monitor the circuit elements of the two firing circuits as much a possible for diagnostic testing purposes.